It pertains more particularly to packaging which comprises a generally rigid formed tray and a generally more flexible sheet material adhered to the tray to form a closing and sealing lid. In such packages, the packages typically open by peeling the lid away from the formed tray.
The use of formed trays and peelable lids for those trays has, in general, been known for some time. For example, jellies, jams, and dressings are commonly packages in individual servings for institutional use in formed trays with peelable sheet material used for lids sealed to the trays. More recently, larger trays and accompanying lids have been used in the packaging of complete servings of prepared food, or individually packaged complete meals. Other uses for such packaging can, of course, be found.
This invention pertains most particularly to those packages which are to be subjected to relatively severe processing conditions either at the point where the package is being filled and sealed or at the point where the package and/or its contents is to be used. Such packaging typically encounters substantial heat at the point where the package is filled and sealed, and may be subjected to significant heat at the point where the package is used. At the point of end use, the package may be reheated. At the point of packaging, the package may be subjected to, for example, hot filled product at approximately 190.degree. F. Processing at the point of packaging may also include processing of the package at, for example, 250.degree. F. or more.
To the extent packaging can be designed which will withstand such processing and use conditions at the point of packaging and/or the point of use, these packages find substantial applications, particularly in the packaging of food products.
In the packaging of food products it is known to provide, in the packaging, certain materials which will protect the contained food products from the permeation of substances into the package from the outside environment. Substances which can have undersirable affects on the contained product are, for example, oxygen, moisture, contaminants, and light. It is especially important, in some applications, to desirably protect the package contents from oxygen, moisture, and light, in addition to typical containing and protecting functions common to most packaging.
Sheet materials for providing these desired protective properties are known and available, and are commonly used in some applications of packaging materials. For example, moisture penetration may be prevented by the use of certain of the olefin polymers such as the polyethylenes, ethylene copolymers, and polypropylenes. These are only exemplary of the known moisture barrier materials. Oxygen penetration can be successfully impeded through the use of various of the vinylidene chloride copolymers, the amide polymers, and the vinyl alcohol polymers.
Light penetration may be prevented by the use of, for example, metal foils, papers, and pigmented polymers.
It should be understood that the description herein of the various ways of preventing the entrance of unwanted factors from the outside environment is only indicative of the conventional art. The recitation herein is not seen to be, and is not intended to be, exhaustive, but rather is exemplary of the existing technology.
As used herein the word "polymer" generally refers to homopolymers and copolymers unless otherwise specifically stated.
A particular problem is encountered in providing packaging for the severe process conditions of, for example, retort processing or the end use environments, in that there are only a limited number of packaging materials which can be used to provide the combined properties of (i) barrier functions, (ii) the ability to survive the severe processing and use conditions, (iii) safety for use in the packaging of food, and (iv) holding the package closed and containing the product.
Of special concern is providing a heat sealable layer on the exterior surface of the flexible sheet material which forms the lid of the package, and which can be heat sealed to, for example, the peripheral flange of the tray and subsequently be removed with a moderate amount of force. The lid cannot, however, be so loosely sealed to the tray that it is inadvertently or otherwise undesirably released from the tray prior to an intentional opening of the package. Thus are there the requirements to be able to seal the package tightly closed in combination with providing a readily openable package. The closure must provide the necessary protection from the outside environment while the package is intended to be closed. The package should be susceptible to being opened with a moderate amount of force by a smooth and constant pulling. The opening should be smooth and steady when a constant opening force is applied, such that no jorky motions or sudden releases of adhesion are encountered. Susceptibility to leaking through the seal area is unacceptable.
It is well known, particularly from the technological work done on retort packaging, that polypropylene is useful as a sealant material where a high temperature environment is contemplated, such as retort processing. Applicants are aware of the use of sheet materials which employ polypropylene and propylene ethylene copolymers as the sealant layer in tray lids. A problem with using polypropylene or its copolymers as the sealant material is that the peel force required to remove the lid from the tray is excessively high. This problem has been recognized in the industry and certain modifications of the polypropylene have been made in attempts to reduce the high level of adhesion between the polypropylene sealant layer and the flange of the tray. In one case, ethylene vinyl acetate (EVA) has been added to the polypropylene in a quantity estimated at 3 to 5 percent by weight. Japanese Patent Open number 77465; 1982 teaches a blend of a propylene ethylene copolymer, a high density polyethylene, and EVA. That document teaches that polypropylene and high density polyethylene have no compatibility, and need the addition of the ethylene vinyl acetate.
Japanese patent open 1672; 1983 teaches a blend of polypropylene and polyethylene as the sealant layer. The polyethylene is defined as having a density below 0.935.
Another reference which teaches blending of material into polypropylene is U.S. Pat. No. 4,189,519 Ticknor, which teaches the blending of an ethylene ester copolymer into the polybutylene to modify its peel properties.
U.S. Pat. No. 4,183,845 teaches blends of polypropylene and polystyrene as a coating composition for use on paperboard.
It is an object of this invention to provide novel polymeric compositions, novel sheet materials, and novel packaging made from those compositions and sheet materials. The sheet materials in general have the capability to be heat sealed to preformed generally rigid trays, and to withstand retort processing conditions of up to 250 degrees Fahrenheit for approximately 30 minutes. The package may subsequently be opened by peeling of the novel sheet material from the tray, accompanied by cohesive failure of a layer within the sheet material, and wherein the cohesive failure is limited to an area generally confined to that area of the sheet material which functions to form the heat seal about the tray. Thus this invention is not directed at nor concerned with cohesive failures which propagate to areas of the package lid which do not in general form a portion of the seal between the sheet material and the remainder of the package such as the tray.
It is a specific object of the invention to provide novel compositions of matter which are two component blends of polyethylene having a density of at least 0.940 and propylene polymers, either homopolymers or copolymers.
It is another specific objective of the invention to provide novel sheet materials, using as a layer thereof, novel blends of high density polyethylene and polypropylene polymer.
It is yet another specific object of the invention to provide novel packaging made from the sheet materials of the invention, the packaging having retort processing capability in combination with peelable seals whose peeling is accompanied by cohesive failure of one of the layers of the sheet material, the layer exhibiting the cohesive failure being a blend of high density polyethylene and a propylene polymer.